It's more apathy than anything else. Leaving it where they last parked it is the path of least resistance and when that value is meaningless to you that's all you'll consider. Honestly where I went to school they were just as likely to leave the keys in it or among the shit they left in their apartment. Compared to the clapped out junkers the cops and towing companies usually had to auction a <4 year old car with low miles is more than welcome.

Here's kind of a more relatable example. I live somewhat close to beaches that have decent weather pretty much year round. Tons of people from up north rent out places here and they'll spend a little bit of money on a boogie board, skimboard, or something like that. 9/10 times it's not worth the hassle of bringing it home, storing it, and remembering it next year (at the earliest). So it's left at the rental and the landlords probably just stuff it in a closet as free-to-use for future guests. Yeah it's arguably douchey behavior but it's mostly harmless.